1. Field of the Invention
The present invention relates to photo-sensitive semiconductor devices, such as photo-diodes for integrated image sensors.
2. Description of the Related Art
The quantum efficiency of an image sensor is defined as the ratio of the number of collected photoelectrons (or photo-holes) to the number of incident photons. (Although the following discussion refers primarily to photo-electrons, those skilled in the art will understand that analogous teachings apply to photoholes.) An ideal image sensor without internal amplification has a quantum efficiency of 1.0, where each incident photon results in a single collected photo-electron. In real-world applications, however, certain effects prevent real image sensors from attaining ideal quantum efficiency.
FIG. 1A shows a schematic top view of a typical photo-diode 100 that is part of a conventional integrated image sensor. FIG. 1B shows a schematic cross-sectional view of photo-diode 100. Photo-diode 100 comprises an N+ region 102 formed within a Pxe2x88x92 substrate 104, only a portion of which is represented in FIGS. 1A-B. As shown in FIG. 1A, region 102 has a rectangular layout in which each interior angle 106 is a right angle (i.e., 90 degrees). In addition, region 102 has four side-walls 108 and a bottom 110.
In operation, an electrical bias is applied between region 102 and substrate 104 to provide photosensitive depletion regions, also referred to as photo-junctions (not shown), at the interfaces between region 102 and substrate 104 (i.e., along side-walls 108 and bottom 110). When a photon of appropriate wavelength is absorbed within a photo-junction, an electron-hole pair is generated and then separated by the applied electrical bias into a xe2x80x9cfreexe2x80x9d photo-electron and a xe2x80x9cfreexe2x80x9d photo-hole. Ideally, each photo-electron is collected by the sensor electronics (not shown) to form part of the photo-electric signal generated by the illuminated photodiode.
Depending on the particular application, one of the problems associated with image sensors is related to the fact that photons having different wavelengths have different absorption lengths (i.e., the distances that photons typically penetrate through the photo-diode structures before being absorbed). For example, in a typical image sensor designed for visible light, photons having longer wavelengths (e.g., corresponding to red light) have a larger absorption length than photons having shorter wavelengths (e.g., corresponding to blue light). As such, a higher fraction of incident blue photons are absorbed within the photo-diode structure before reaching a photo-junction than the fraction of incident red photons. Photons that are absorbed before reaching a photo-junction produce less efficiently collected photo-electrons. As such, in a typical image sensor, the quantum efficiency of the sensor varies as a function of the frequency of the incident light, with a higher quantum efficiency for red light than for blue light. This results in an image sensor having non-uniform spectral sensitivity, which is a disadvantage in many imaging applications.
Another effect that limits the quantum efficiency of an image sensor is leakage. Leakage occurs when the collected photo-charge crosses the junction before the signal can be read. In addition to reducing quantum efficiency, such leakage can also result in unacceptably high levels of offset and dark noise, especially as technology shrinks and image sensors become more sensitive.
Embodiments of the present invention are configured to address problems including (a) non-uniform sensor spectral sensitivity and (b) leakage, each of which limits the overall quantum efficiency of the resulting photo-sensors. In particular, for example, some photo-diodes in accordance with the present invention have geometries with (1) relatively large total interface areas and (2) non-rectangular layouts in which all interior angles are larger than 90 degrees. The large total interface area can improve the uniformity of sensor sensitivity by providing more photo-junction volume close to the surface of the photo-diode, thereby enabling a greater fraction of incident photons having smaller absorption lengths to be absorbed within photo-junctions and produce collected photo-electrons. A non-rectangular layout with all interior angles greater than 90 degrees can decrease leakage by decreasing the electric field strengths as well as the physical stresses along the non-horizontal (e.g., vertical) edges of the photo-diode. Whether implemented together or independently, these features tend to improve the overall quantum efficiency of the corresponding image sensors.
In one embodiment, the present invention is an integrated circuit having a photo-sensing element comprising a first region formed within a substrate, wherein a vertical cross-section of the first region defines a set of interior side-wall interfaces, a set of exterior side-wall interfaces, and a set of one or more bottom interfaces, such that a horizontal line can be drawn through the vertical cross-section that would cross a first exterior surface, followed by a first interior surface, followed by a second interior surface, followed by a second exterior surface.
In another embodiment, the present invention is a method for fabricating an integrated circuit having a photo-sensing element comprising the steps of (a) forming a first region within a substrate, wherein a vertical cross-section of the first region defines a set of interior side-wall interfaces, a set of exterior side-wall interfaces, and a set of one or more bottom interfaces, such that a horizontal line can be drawn through the vertical cross-section that would cross a first exterior surface, followed by a first interior surface, followed by a second interior surface, followed by a second exterior surface; and (b) forming one or more additional structures on the substrate in conjunction with the first region to fabricate the photo-sensing element.